In the field of mobile or wireless communication, the term “wireless device” or just “device” for short, is often used and will be used in this disclosure to represent any wireless communication entity capable of radio communication with a radio network by sending and receiving radio signals, such as e.g. mobile telephones, tablets and laptop computers. Another common term in this field is “User Equipment, UE”. A wireless device in this context could also be a machine-to-machine type of device operating automatically such as a sensor, counter or measuring entity which is configured to send reports over the radio network e.g. at certain intervals or upon certain events.
Further, the term “network node”, sometimes also referred to as a base station, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc., is used here to represent any node of a radio network that is arranged to communicate radio signals with wireless devices. The network node described here may, without limitation, be a so-called macro base station or a low power base station such as a micro, pico, femto, Wifi or relay node, to mention some customary examples. Throughout this disclosure, the terms “base station” and “UE” could alternatively be used instead of network node and wireless device, respectively.
In a typical radio network for wireless communication, a multitude of different wireless devices are frequently communicating by transmitting data over uplink data channels to network nodes serving different cells in the radio network. The normal procedure is that a wireless device intending to transmit data must first transmit a scheduling request to its serving network node and then await an uplink grant from the network node which specifies a radio resource, e.g. in terms of subframes on a certain frequency and/or channel, being reserved for the data transmission. This procedure is sometimes referred to as access reservation.
However, since it has become more and more common that many wireless devices frequently need to transmit only quite small amounts of data at irregular and unpredictable intervals, the above-described access reservation procedure with request and grant before each data transmission becomes a burden and creates a great “overhead” of signaling and delays for both the wireless device and the network node to handle. The access reservation procedure thus requires substantial amounts of control signaling and processing for communicating relatively small amounts of data, and also causes latency in the data communication. Another drawback is that power consumption is high in the wireless device. As a result, it has been suggested in the Third Generation Partnership project, 3GPP, that contention-based uplink transmission can be employed instead of the above-described access reservation for such data transmissions. Some examples of how contention-based uplink transmissions can be employed are described in WO 2010057540 A1.
Contention-based uplink transmission means that any wireless device can transmit data to a serving network node on a common radio resource at the risk of collision when two or more devices happen to transmit simultaneously. This works well as long as there are only a limited number of wireless devices in the cell and collisions occur rarely. Some network nodes are equipped with advanced receivers capable of so-called Multi-User Detection, MUD, so that transmissions received from more than one wireless device at the same time can be decoded.
The network node receiving such a contention-based uplink transmission is able to determine the transmitting device and decode the transmitted data by performing so-called “blind decoding” based on some identifier of each potential transmitting device. This means that the network node must attempt to decode the received transmission based on one identifier at the time, thus performing blind decoding across all devices in the cell by going through the identifiers, one by one, of all wireless devices that may potentially have transmitted the received data.
Examples of how blind decoding of a received transmission can be performed are described in US 20090154607 A1.
It is however a problem that when a network node is serving a great number of wireless devices, it will need to perform blind decoding across all of these wireless devices in order to decode a received contention-based uplink transmission. FIG. 1 illustrates a communication scenario where the feature of contention-based uplink transmission is employed e.g. on a specific radio resource or channel reserved for such transmissions. This figure shows that multiple wireless devices D1-D6 are being served by a network node 100 in a cell 102. In reality, there may be a much larger number of devices being served by the same network node and this figure only illustrates this schematically. According to conventional procedures, the network node 100 performs blind decoding across all of these wireless devices D1-D6 in order to identify and decode a received contention-based uplink transmission, which is therefore a quite complex and processing-heavy operation when the number of devices is large. The risk for failed decoding also increases with the number of potential wireless devices present in the cell such that the transmitting device have to try again by re-transmitting its data.
If the devices D1-D6 also can use different transmission formats, or Modulation and Coding Schemes, MCSs, each potential wireless device needs to be tested by the network node 100 for all possible transmission formats or MCSs, thereby requiring the network node to perform the blind decoding not only across a large number of devices but also over all possible transmission formats, resulting in a huge number of possible hypotheses for each received transmission.